1. Field of the Invention
The present invention relates to a process for preparing particulate, mixed oxides using an alumina hydrate as one of the starting materials. More particularly, the present invention relates to an improved process for producing mullite.
2. Description of the Background
Mixed metal oxides are finding increasing use in the manufacture of precision ceramic parts for high tech ceramics, solid state electronics and other high technology fields. The starting materials for such end products are generally sub-micron, de-agglomerated mixed metal oxide powders, e.g. mullite. Mullite is an aluminum silicate having the formula 3Al.sub.2 O.sub.3 .multidot.2SiO.sub.2 (Al.sub.2 O.sub.3 /SiO.sub.2 =1.5) that forms a solid solution with aluminum oxide when the molar ratio of Al.sub.2 O.sub.3 /SiO.sub.2 is in a range of 1.50 to 2.87 (Ceramic International, 9 (4), 107-113 (1983) by J. A. Pash). Mullite is recognized as a useful material in ceramic applications where high temperature stability is required. Mullite is also resistant to thermal shock making it useful in critical applications. To obtain the desired properties, e.g. low sintering temperature, it has been found necessary to utilize synthetic mullite where the homogeneity and microstructure of the mullite can be controlled. Specifically, it is desirable to produce mullite having sub-micron particle size.
Synthetic mullites are currently produced by a variety of methods including coprecipitation, sol-gel, vapor phase reaction, and thermal diffusion. Of the above mentioned, the sol-gel technique is currently the most widely practiced method. In the sol-gel method, a colloidally dispersed boehmite sol is combined with a dispersed colloidal silica sol to obtain an intimate mixture. The mixture is then dried, milled to reduce particle size, fired to convert the amorphous mixture to mullite, and then milled again to reduce the particle size of the finished powder. Generally speaking, the boehmite sol employed in the prior art methods consists of small size boehmite crystals referred to as "pseudoboehmite".
Techniques for producing mullite by sol-gel techniques are disclosed, for example, in German Patent DE 3,618,576, and Japanese Kokai No. SHO 63(1988-144111). Additionally, in U.S. Pat. No. 4,679,928, there is disclosed a method of forming a water-dispersible boehmite colloidal sol.